3 Answers
3

The closest answer is (E) 5300. Great intuition on how to find simple approximations, but you forgot to multiply by $1.2$! Also note that $1.44<1.5$ and $256<266$, so you know the true answer must be above the discovered approximation, leaving only the last answer.

Edit: Note that the only approximation I used here is $\sqrt{399}\approx \sqrt{400}$ so the result will differ by a factor of $\frac{\sqrt{399}}{20}$. This can be quickly approximated too, $\frac{\sqrt{399}}{20} = \sqrt{1-1/400} \approx 1 - 1/800 =0.9987 $.